Calibration methods and circuits to calibrate drive current and termination impedance

ABSTRACT

Described are on-die termination (ODT) systems and methods that facilitate high-speed communication between a driver die and a receiver die interconnected via one or more signal transmission lines. An ODT control system in accordance with one embodiment calibrates and maintains termination resistances and drive currents to produce optimal output swing voltages. Comparison circuitry employed to calibrate the reference resistance is also used to calibrate the drive current. Termination elements in some embodiments are divided into two adjustable resistive portions, both of which are designed to minimize capacitive loading. One portion is optimized to produce a relatively high range of adjustment, while the other is optimized for fine-tuning and glitch-free switching.

BACKGROUND

High-speed data communication integrated circuit (IC) dies are known toinclude both drivers and receivers. The driver of one such IC connectsto the receiver of another via one or more signal transmission lines.Both the driver and receiver circuits include termination elements thatattempt to match the characteristic impedance of the transmission lineto the output impedance of the driver and input impedance of thereceiver, as impedance mismatches degrade signal quality andconsequently reduce communication speed and reliability.

Some conventional communication systems employ control systems thatcalibrate the impedance of on-chip termination elements for improvedimpedance matching. For a detailed discussion of one such system, seeU.S. Pat. No. 6,418,500 entitled “Feedback Control for TerminationAdjustment,” which issued on Jul. 9, 2002, to Gai et al.

Some high-performance communication systems employ current-modeswitching for improved speed performance. Such systems includecurrent-mode drivers, which convey digital signals by modulating currentbetween values representative of different logic levels. The levels ofcurrent used to express logic levels are carefully calibrated andcontrolled to obtain rapid switching and low power consumption. Thefollowing issued patents describe conventional circuits for calibratingthe drive current for current-mode drivers: U.S. Pat. No. 5,254,883entitled “Electrical Current Source Circuitry for a Bus,” which issuedon Oct. 19, 1993, to Horowitz et al., and U.S. Pat. No. 6,462,591entitled “Semiconductor Memory Device Having a Controlled Output DriverCharacteristic,” which issued on Oct. 8, 2002, to Garrett et al.

The aforementioned calibration techniques for setting and maintainingappropriate termination resistances and drive currents work well in manyapplications. High-speed circuits are in a very competitive market,however, and must achieve ever greater performance levels to satisfycustomer demand. There is therefore a need for area- and power-efficientcommunication circuits that exhibit improved speed performance.

SUMMARY

The present invention is directed to efficient on-die termination (ODT)systems and methods that facilitate high-speed communication between adriver die and a receiver die interconnected via one or more signaltransmission lines. An ODT control system in accordance with oneembodiment calibrates and maintains the termination resistances anddrive currents of the driver die to produce optimal output swingvoltages on the signal transmission lines. A similar ODT control systemcalibrates and maintains the termination resistances of the receiverdie. The ODT control systems rely upon external voltage and resistancereferences for termination resistance and drive-current calibration.

An ODT control system in accordance with one embodiment calibrates drivecurrent by matching the drive current to a reference current drawnthrough a precision resistor, and calibrates termination resistance bymatching on-die termination elements with the same external resistor.Comparison circuitry employed to match the reference and drive currentsis also used to match the reference and ODT resistances. This sharing ofresources saves power and die area. In addition, offset errorsintroduced by shared components impact the resistance and currentcalibrations in opposite ways, and thus tend to cancel one another.

Termination elements in some embodiments are divided into two adjustableresistive portions, both of which are designed to minimize capacitiveloading. One portion is optimized to produce a relatively high range ofadjustment, while the other is optimized for fine-tuning and glitch-freeswitching. Active ODT control systems can calibrate the first portionbefore respective drive and receive circuitry begin communicating, andcan then employ the second portion to account for resistive drift thatoccurs due to supply-voltage and temperature fluctuations. In oneembodiment, the second portion employs a serial, thermometer-codedvoltage divider to provide glitch-free switching between ODT resistancelevels. In another embodiment, the second portion employs an analogvoltage divider. Either embodiment can be used to periodically calibrateODT resistance without introducing undesirable resistive glitches.

This summary does not limit the invention, which is instead defined bythe allowed claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of a data communication system 100 inaccordance with one embodiment.

FIG. 2 depicts a termination resistance and drive current calibrationsystem 200 that includes ODT control system 130, reference resistor 175,and reference source 170 of FIG. 1.

FIG. 3 depicts a process 300 employed by control system 130, as detailedin FIG. 2, to calibrate reference ODT resistor 230 and ODT current Iodt.

FIG. 4 depicts a communication system 400 in accordance with anotherembodiment.

FIG. 5 details an embodiment of calibration system 200 of FIG. 2.

FIG. 6 depicts an ODT termination element 600 in accordance with oneembodiment.

FIG. 7 depicts a termination element 700 in accordance with anotherembodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a data communication system 100 inaccordance with one embodiment. Communication system 100 includes afirst integrated circuit (IC) die 105 connected to a second IC die 110via a number of signal transmission lines 115. Die 105 includes aplurality of identical driver circuits 120 and an on-die-termination(ODT) control system 130. Control system 130 establishes and maintainsthe termination resistances and drive currents of driver circuits 120 toproduce optimal output swing voltages.

Die 110 includes a plurality of identical receiver circuits 125, each ofwhich receives signals from a corresponding one of driver circuits 120of die 105. Die 110 includes an ODT control system 187 that establishesand maintains the termination resistance applied to incoming signals tooptimize the swing voltages of those signals.

Each of dies 105 and 110 may be any of myriad types of processing chipscapable of communicating electrical signals. Typical examples include ICdies that communicate via parallel or serial bus interfaces.Transmission lines 115 may be, for example, a wire, a cable, a trace ona printed-circuit board, a chip etching, a waveguide, or other mediumthat allows for transmission of signals.

For simplicity, die 105 and die 110 are presumed to support respectivedriver and receive circuitry; in other embodiments, however, die 105additionally supports receive circuitry, and die 110 additionallysupports transmit circuitry. Such embodiments can use eitherunidirectional or bidirectional signal lines, as is well known to thoseof skill in the art. Also important, the depicted embodiment isdescribed in connection with a typical case in which two diescommunicate signals via external lines, but other embodiments optimizesignal line termination resistance, drive current, and signal swing toimprove communication speed between circuits that exist on the same die.Driver circuits 120 are identical, as are receiver circuits 125. Thefollowing description is limited to the topmost driver/receiver pair forbrevity.

Driver circuit 120 includes a driver 135 receiving an input data streamrepresented as a pair of complimentary input signals Din0 and Din0 b,which are provided on complimentary driver input nodes having the samedesignations. (As with other designations herein, Din0 and Din0 b referboth to signals and their corresponding nodes; whether a givendesignation refers to a signal or a node will be clear from thecontext.) Driver 135 additionally includes a pair of driver outputterminals 150 and 155 that connect to signal transmission lines 115 viaIC pads (not shown). Driver output terminals 150 and 155 conveycomplimentary output signals derived from the input data stream. (Unlessotherwise defined, terminals, lines, conductors, and traces that carry agiven signal fall under the umbrella term “node”; in general, the choiceof a given description of a circuit node is a matter of style, and isnot limiting.)

Driver circuit 120 includes a pair of termination elements 145 and adigital-to-analog converter (DAC) 140. Each of driver output terminals150 and 155 connects to a reference voltage Vio via a respectiveadjustable termination element 145, and each termination element 145receives digital ODT calibration signals from ODT control system 130 viaan ODT calibration port 165. Driver circuit 120 additionally includes acurrent control port 160, which receives digital current-calibrationsignals from control system 130. DAC 140 converts these digital signalsinto a current-control voltage that controls the level of drive currentprovided by driver 135. By controlling both the termination resistanceand the drive current of driver circuit 120, ODT control system 130optimizes the swing voltages on driver output terminals 150 and 155, andconsequently reduces noise and improves speed performance.

The resistance values of termination elements 145 and the drive currentprovided by driver 135 vary with process, voltage, and temperature. ODTcontrol system 130 therefore relies upon precise reference components toestablish appropriate termination resistances and drive currents. In thedepicted embodiment, die 105 is connected to a first reference voltageVr1 through an external voltage source 170 and an external precisionresistor 175. Reference source 170 conveys a second reference voltageVr2 to ODT control system 130. The difference between reference voltageVr1 and Vr2 is used to establish an appropriate swing voltage. Referenceresistor 175 conveys a reference-resistor voltage Vrr to ODT controlsystem 130, acting as a reference resistance against which to compareand adjust termination elements 145. A calibration process forestablishing the appropriate resistance through termination elements 145and drive currents provided by drivers 135 is described below inconnection with FIGS. 2 and 3.

Receiver circuit 125 within die 110 includes a differential amplifier185 connected to a pair of complimentary signal transmission lines 115via respective receiver input terminals 186 and 189. Each of inputterminals 186 and 189 connects to reference voltage Vio via a respectivetermination element 180. Termination elements 180 are similar toelements 145 in this embodiment, and are controlled via a line ODT froman ODT control system 187 similar to system 130. Control system 187compares the values of termination elements 180 with an off-chip,precision reference resistor 195 to account for process, voltage, andtemperature variations that affect the resistance values of terminationelements 180. ODT control system 187 is also capable of calibratingdrive current, though no drivers are shown in FIG. 1. Wheredrive-current calibration is desired, control system 187 connects to anexternal reference voltage 190. Receiver circuit 125 receives theincoming data stream on complimentary input terminals 186 and 189 andpresents a resulting data to the interior of IC die 110 viacorresponding data output node Dout0.

FIG. 2 depicts a termination resistance and drive current calibrationsystem 200 that includes ODT control system 130, reference resistor 175,and reference source 170 of FIG. 1. (In general, the first digit of agiven numerical designation indicates the Figure in which the indicatedelement was introduced. For example, reference resistor 175 wasintroduced in FIG. 1.)

Reference source 170 and reference resistor 175 are external, precisionelements connected to system 130 via a pair of pads 201 and 202. Controlsystem 130 includes a comparator 215, an analog multiplexer 220, areference ODT resistor 230, and a current source 240. Control system 130calibrates the resistance value Rodt of resistor 230 to match thereference resistance Rref of external precision resistor 175. Controlsystem 130 also matches the current Iodt through resistor 230 with areference current Irr through reference resistor 175 to produce adesired voltage drop across ODT resistor 230. ODT counter 245 stores adigital value representative of the offset required to calibrate ODTresistor 230, and a current-control counter 250 stores a digital valuerepresentative of the offset required to calibrate current source 240.The digital values from counters 245 and 250 are then conveyedthroughout die 105 (FIG. 1) to calibrate similar or identicaltermination elements and current sources within driver circuits 120.

FIG. 3 depicts a process 300 employed by control system 130, as detailedin FIG. 2, to calibrate reference ODT resistor 230 and ODT current Iodt.First, at step 305, current-calibration signal ICAL1 b, an active-lowsignal, is asserted. Analog multiplexer 220 thus selects secondreference voltage Vr2 for comparison with reference voltage Vrr fromresistor 175. The inverse of signal ICAL1 b, ICAL1, enablescurrent-control counter 250 to increment and decrement in response tooutput signals from comparator 215.

Current source 240 pulls reference current Irr through referenceresistor 175. Per Ohm's law, reference voltage Vrr equals referencevoltage Vr2 when the product of reference current Irr and referenceresistance Rref equals the swing voltage Vsc across reference source 170(i.e., when Irr*Rref=Vsc). Comparator 215 compares voltage Vrr withvoltage Vr2 (decision 310), and adjusts the contents of current-controlcounter 250 as necessary to render voltage Vrr equivalent to voltage Vr2(step 315). For example, if comparator 215 determines that Vrr is lowerthan Vr2, comparator 215 increments counter 250 to reduce current Irr,and consequently increase the voltage Vrr. The comparison and adjustmentof steps 305, 310, and 315 continue until Vrr and Vr2 are substantiallyequal. At this point, current counter 250 stores a count CCNT thatoffsets the current Irr provided by current source 240. ODT current Iodtis substantially equal to reference current Irr, so steps 305, 310, and315 place current Iodt at or near the expected operating level.

The granularity of current-control count CCNT introduces someuncertainty as to the exact reference current Irr and correspondingIodt. Employing more register bits and associated bus lines for CCNTreduces this uncertainty at a cost of increased overhead.

The next portion of calibration process 300 sets resistance Rodt ofresistor 230 substantially equal to reference resistance Rref ofprecision resistor 175 (i.e., Rodt≈Rref) by adjusting the count incounter 245. The first current-calibration signal ICAL1 b is deasserted(held to a logic one), causing multiplexer 220 to apply voltage Vodtfrom resistor 230 to the second input terminal of comparator 215.Deasserting ICALb also disables counter 250, freezing current controlsignal CCNT to hold Iodt and Irr constant. A termination calibrationsignal TCAL to counter 245 is asserted, enabling counter 245 toincrement and decrement in response to output signals from comparator215.

In step 320, comparator 215 compares Vrr and Vodt. ODT counter 245increments or decrements in response to the output of comparator 215,altering the resistance Rodt through ODT resistor 230. The input voltageVio to termination resistor 230 is connected to off-chip voltagereference Vr1, and is therefore substantially equivalent to voltage Vr1.Because the currents Irr and Iodt are substantially identical, Ohm's lawprovides that voltage Vodt is substantially equal to voltage Vrr ifresistance Rodt of resistor 230 matches reference resistance Rref ofprecision resistor 175. Comparator 215 thus adjusts the contents ofcounter 245 (step 330) until decision 325 determines that voltages Vrrand Vodt are substantially equal. The granularity of ODT-control countODT introduces some uncertainty as to the exact reference resistanceRodt. This uncertainty can be improved by employing more register bitswithin counter 245, but this improvement comes at a cost of increasedoverhead.

Input/output voltage Vio, being distributed on chip, may vary by somesmall amount from external reference voltage Vr1. The resultinguncertainty combines with other uncertainties, such as those resultingfrom the measurement granularities of counters 245 and 250, to producestill greater uncertainty. A second current-calibration sequencerecalibrates ODT current Iodt using the calibrated ODT resistor 230 andinput voltage Vio to reduce this uncertainty.

Returning to FIG. 1, each signal transmission line 115 includes anassociated pair of termination elements 145 and 180 connected inparallel between Vio and respective ends of the signal transmissionline. To duplicate this arrangement, current control system 130 includesa transistor 255 that shorts reference resistor 175 and ODT resistor 230in response to a second current-calibration signal ICAL2, effectivelyconnecting resistors 175 and 230 in parallel between node Vio (recallVr1 is an off-chip version of Vio) and node Vodt. Because voltages Vr1and Vio and resistances Rref and Rodt are substantially equal, thecurrent between interconnected nodes Irr and Iodt is expected to bequite low.

In step 335, signal TCAL is deasserted and both current-control signalsICAL1 b and ICAL2 are asserted. Counter 245, and thus resistance Rodt,is thus held constant as comparator 215 once again compares Vr1 with Vrr(step 340). Counter 250 increments or decrements in response to outputsignals from comparator 215 (step 350). The calibration process 300 iscomplete when system 130 voltages Vr2 and Vrr are substantially equal(step 345). The resulting offsets held in counters 245 and 250 aredistributed on buses ODT and CCNT to drivers and receivers on the samedie to establish appropriate ODT resistances and drive currents in themanner described above in connection with FIG. 1.

The current and resistance calibration steps are carried out usingshared resources, including the low-offset, high-gain comparator 215.This sharing of resources saves power and die area. In addition, offseterrors introduced by shared components impact the resistance and currentcalibrations in opposite ways, and so tend to cancel one another. Forexample, a comparator error that results in a slightly high ODTresistance Rodt produces a slightly low ODT current Iodt. Rodt and Iodtare multiplied to produce the desired swing voltage, so the increase inRodt tends to cancel the decrease in Iodt.

FIG. 4 depicts a communication system 400 in accordance with anotherembodiment. System 400 includes components in common with communicationsystem 100 of FIG. 1, like-elements being the same or similar. System400 includes first and second bus interfaces 405 and 410, each of whichis typically a portion of a respective IC die, interconnected viaparallel bus 417. One driver circuit 415 of interface 405 and acorresponding receive circuit 420 of interface 410 are detailed: theremaining driver circuits 415 and receive circuits 420 are representedas simple boxes for brevity.

Driver circuit 415 includes a pair of termination elements 425 (ODTresistors) connected between reference voltage Vio and respective onesof a pair of signal transmission lines. Driver circuit 415 additionallyincludes a current-controlled amplifier 418, which in turn includes apair of input transistors 430 and a current-source transistor 435. Eachof transistors 430 connects between one of termination elements 425 anda power-supply terminal GND via transistor 435. A DAC 440 within drivercircuit 415 controls the current through transistor 435 by applying acontrol voltage CV to the gate of transistor 435, and consequentlycontrols the maximum voltage drop across termination elements 425.

ODT control system 130 derives offsets ODT and CCNT to calibratetermination resistance and drive current in the manner described abovein connection with FIGS. 2 and 3. DAC 440 derives control voltage CVfrom the current-control offset CCNT so that the drive current ofamplifier 418 equals the ODT current Iodt calibrated in process 300.Termination elements 425 are fabricated to be identical to reference ODTresistor 230, and so respond in kind to offset signal ODT from controlsystem 130. In short, control system 130 calibrates a referencetermination element and a reference drive current by establishingappropriate ODT and CCNT counts, and these counts are applied to drivercircuits 415 on the same die to establish and maintain the correcttermination resistances, drive currents, and voltage swings.

Receive circuit 420 does not generate a drive current, but includes apair of termination elements 445 Like termination elements 425 of drivercircuit 415, termination elements 445 are calibrated by an offset ODTfrom the associated ODT control system 187. Though not shown, otherembodiments include interconnected transceivers, each of which includeboth drive and receive circuitry. ODT control system 187 can be used toestablish appropriate termination-element resistance, drive current, andswing voltage levels in such embodiments. Transmission lines can bebidirectional in systems that include interconnected transceiver pairs.

FIG. 5 details an embodiment of calibration system 200 of FIG. 2,like-numbered elements being the same or similar. In this embodiment,ODT control system 130 includes additional components, many of which areincluded to more closely match the behavior of the reference elementsused for calibration with the termination elements and drivers to becalibrated. The example assumes the first reference voltage Vr1 andinput/output voltage Vio are each 1.8 volts, reference resistance Rrefis 50 ohms, swing calibration voltage Vsc is 350 millivolts, andreference current Irr is 7 milliamps; however, these values areillustrative and not limiting.

Terminal Vrr of comparator 215 connects to reference resistor 175 via apass gate 505. Pass gate 505 is included to minor the behavior of a pairof pass gates that are combined to form analog multiplexer 220. Passgate 505 offers the same resistance as multiplexer 220, and thus cancelsvoltage differences on the input terminals of comparator 215 that wouldotherwise introduce errors. Complementary outputs from comparator 215feed an ODT counter 506 and current counter 507 via a digital filter 509and sampling latch 510. Sampling latch 510 periodically samples theoutput of filter 509. If enabled, one of counters 506 and 507 incrementsor decrements in response to the output from sample latch 510 to adjustthe count on one of buses ODT<8:0> or CCNT<6:0>. Comparator 215, filter509, and latch 510 are shared to save power and die area, and to reduceoffset errors in the manner discussed above in connection with FIG. 2.

Current source 240 includes a resistor 515, a reference current source520, a DAC 525, and a pair of current-source transistors 530. Resistor515 is a replica of ODT resistor 230, but may be of a different size orvalue. Resistor 515 is formed using the same process sequence asresistor 230, and so varies in the same manner with process, voltage,and temperature. Current source 520 produces a reference current Irefproportional to the value of resistor 515. DAC 525 uses referencecurrent Iref to derive a control voltage CCREF proportional to theresistance of resistor 515, and consequently proportional to Rodt.Current-control bus CCNT<6:0> then provides offset values to DAC 525 toadjust control voltage CCREF as necessary to calibrate currents Irr andIodt. In one embodiment, current source 240 provides a range of outputcurrents Irr and Iodt that range from about zero to about twice Iref.Current source 240 begins the calibration sequence in the middle of thisrange (Irr=Iodt=Iref).

Current source 240 includes an additional pair of transistors 535 and540, which serve two purposes: first, transistors 535 and 540 candisable current source 240 to save power when ODT control system 130 isnot in use; second, when ODT enable signal ODTEN is asserted,transistors 535 and 540 combined with transistors 530 emulate thebehavior of transistors 430 and 435 within driver circuit 415 of FIG. 4.Transistors 535 and 540 are the same size (W/L) as transistors 430, andtransistors 530 combined are the same size as transistor 435. Withcurrent calibration signal ICAL2 asserted to forward bias a pair oftransistors 545 and 550, transistors 530, 535, and 540 and reference ODTresistor 230 and reference resistor 175 together emulate the behavior oftransistors 430 and 435 and termination elements 425 of driver circuit415. The counts within counters 506 and 507 employed to establishappropriate termination resistances and drive currents within system 130are therefore representative of the counts required to properly adjustthe termination elements and drive currents depicted in FIG. 4.

FIG. 6 depicts an ODT termination element 600 in accordance with oneembodiment. Termination element 600 can be used in place of ODT resistor230 of FIGS. 2 and 5 and in place of termination elements 425 and 445 ofFIG. 4.

Termination element 600 includes a number of OR gates, each of whichincludes one input terminal connected to a high-impedance-selectterminal RHIZ. When asserted, signal RHIZ disables termination element600, providing a high impedance between reference voltage Vio and theassociated transmission line (for termination elements 425 and 445) orto multiplexer 220 (for reference ODT resistor 230). The followingdescription assumes signal RHIZ is deasserted, so the OR gates merelypass whatever signal is provided on their respective second inputterminals.

Termination element 600 includes two separate portions: abinary-weighted portion 605 and a thermometer-coded portion 610. Portion605 includes a resistor network 635 and a collection of PMOS transistors630, the gates of which are connected to the first six lines ODT<5:0> ofcontrol bus ODT<8:0> of FIG. 5. Each PMOS transistor 630 controls thecurrent through a respective resistive path. These resistive pathsprovide binary-weighted resistances from about 50 to 1600 ohms, soportion 605 can produce a wide range of resistances by enabling selectedtransistors 630. (The weighted resistance values can vary widely withprocess, temperature, and voltage variations, however, hence the needfor ODT calibration.)

Transistors 630 are relatively small to reduce the effect of capacitanceon transmission lines 417, so transistors 630 provide a significantpercentage of the resistance through portion 605 (e.g., as much as about30% in one embodiment). The I-V characteristics transistors 630 aresomewhat non-linear, and this non-linearity produces some non-linearityin the resistance through portion 605. The first current-calibrationsequence sets ODT current Iodt close to the operating current beforeadjusting Rodt so the non-linear response of Rodt does not introducesignificant errors in Rodt when Iodt is adjusted during the swingcalibration. The first current-calibration sequence can be omitted ifaccuracy is less important or if the termination elements exhibit morelinear I-V characteristics.

The binary-weighted scheme of portion 605 provides a relatively lowcapacitance, area-efficient means of producing a large number ofpotential resistance values, but can introduce problematic glitches whenswitching between values. For example, incrementing the six low-orderbits ODT<5:0> from 011111 to 10000 to vary the selected resistance fromabout 50 ohms to about 52 ohms turns off the right-most one oftransistors 630 and turns on the remaining transistors 630. Imperfecttiming can produce intermediate resistance values significantly over orunder the desired 52 ohms. Such a resistive glitch can causetransmission errors in systems that dynamically monitor and adjusttermination values to compensate for supply-voltage and temperaturevariations.

Thermometer-coded portion 610 addresses the glitch problem associatedwith portion 610. Portion 610 facilitates fine resistance adjustments byincrementally connecting transistors 620 in parallel with resistiveelements in a voltage divider 615. Thermometer coding means thattransistors 620 are enabled one at a time, in series. The possiblecombinations of signals on lines CCNT<8:6> are 000 (lowest resistance),100, 110, and 111 (highest resistance). Other schemes are possible, andmore or fewer series resistors can be included. Transistors 620 arearranged in series to minimize the effects of capacitance.

Portion 605 is calibrated in the manner described above and the offsetprovided on ODT<5:0> held steady during operation. Portion 610 canlikewise be held steady, or can be modified continuously or periodicallyto compensate for supply-voltage or temperature fluctuations. Likecontrol bits ODT<5:0>, control bits ODT<8:6> can be derived by a commoncalibration circuit and distributed to a number of termination elements;alternatively, control bits ODT<8:6> can be derived separately for eachsignal pad. An ODT-control signal CNTODTb can be deasserted to disableportion 610, while calibrating portion 605, for example.

Thermometer-coded portion 610 is less area efficient than portion 605,but can be incremented or decremented without introducing undesirableglitches. For further discussion relating to combining thermometer andbinary coding, see U.S. Pat. Nos. 6,606,004 and 6,297,759, each of whichis incorporated herein by reference.

FIG. 7 depicts a termination element 700 in accordance with anotherembodiment. Termination element 700 is similar to termination element600 of FIG. 6, like numbered elements being the same or similar.Termination element 700 differs from termination element 600 in thatthermometer-coded portion 610 of termination element 600 is replacedwith an analog resistive element 705 to facilitate fine-tuning oftermination resistance.

The resistance between terminals Vio and Vodt includes the parallelcontributions from portions 605 and 705. Portion 605 is tuned in themanner described above using an ODT count. An integrator composed of acomparator 710 and capacitor 712 then alters the gate voltage of atransistor 707 to alter the resistance through portion 705 such thatvoltages Vrr and Vodt are equal. The gate voltage on transistor 707 in areference ODT termination element can be distributed to similartransistors 707 in termination elements associated with propagatingsignals. Alternatively, each analog portion 705 can be controlledlocally using a dedicated off-chip reference resistor for each pin. Ineither case, portion 605 of termination elements associated with signalconveyance (e.g., elements 425 and 445) can be controlled by distributedODT control signals ODT<5:0>.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example, (1) the externalvoltage and resistance references may be substituted in otherembodiments with on-chip references; (2) the ODT circuits can beconstructed entirely of PMOS transistors, as PMOS transistors behavelike a resistor when operating in the linear region (e.g., with the gateconnected to ground); (3) the thermometer portion 610 (FIG. 6) can havemultiple “legs” connected in parallel; and (4) one or more of thetransistors 620 within thermometer portion 610 can be replaced with asingle transistor controlled by a digital-to-analog converter. Moreover,some components are shown directly connected to one another while othersare shown connected via intermediate components. In each instance themethod of interconnection, or “coupling,” establishes some desiredelectrical communication between two or more circuit nodes, orterminals. Such coupling may often be accomplished using a number ofcircuit configurations, as will be understood by those of skill in theart. Therefore, the spirit and scope of the appended claims should notbe limited to the foregoing description.

1. A termination control circuit comprising: a first node to receive afirst reference voltage; a second node to receive a second referencevoltage; an adjustable resistor exhibiting an adjustable impedanceresponsive to an impedance-control signal; a comparator having a firstinput coupled to the first node to receive the first reference voltage,a second input coupled to the second node via the adjustable resistor,and a comparator output; a transistor disposed in a path between thesecond input and a supply terminal, the supply terminal to receive athird reference voltage; and a counter operatively coupled between thecomparator output and the adjustable resistor, the counter to update theimpedance-control signal responsive to a signal from the comparatoroutput; wherein updating the impedance-control signal adjusts theimpedance between the second node and the second input of thecomparator; and wherein the transistor selectively disables current flowthrough the adjustable resistor.
 2. The termination control circuit ofclaim 1, wherein the counter is operatively coupled between thecomparator and the adjustable resistor via a latch, the latch to capturethe signal from the comparator output.
 3. A termination control circuitcomprising: a first node to receive a first reference voltage; a secondnode to receive a second reference voltage; an adjustable resistorexhibiting an adjustable impedance responsive to an impedance-controlsignal; a comparator having a first input coupled to the first node toreceive the first reference voltage, a second input coupled to thesecond node via the adjustable resistor, and a comparator output; atransistor disposed in a path between the second input and a supplyterminal, the supply terminal to receive a third reference voltage; anda counter operatively coupled between the comparator output and theadjustable resistor, the counter to update the impedance-control signalresponsive to a signal from the comparator output; wherein updating theimpedance-control signal adjusts the impedance between the second nodeand the second input of the comparator, and the counter is operativelycoupled between the comparator and the adjustable resistor via a latch,the latch to capture the signal from the comparator output; and afilter, wherein the latch is operatively coupled to the comparator viathe filter.
 4. A termination control circuit comprising: a first node toreceive a first reference voltage; a second node to receive a secondreference voltage; an adjustable resistor exhibiting an adjustableimpedance responsive to an impedance-control signal; a comparator havinga first input coupled to the first node to receive the first referencevoltage, a second input coupled to the second node via the adjustableresistor, and a comparator output; a transistor disposed in a pathbetween the second input and a supply terminal, the supply terminal toreceive a third reference voltage; and a counter operatively coupledbetween the comparator output and the adjustable resistor, the counterto update the impedance-control signal responsive to a signal from thecomparator output; wherein updating the impedance-control signal adjuststhe impedance between the second node and the second input of thecomparator; and wherein the transistor includes a control terminal, andwherein the signal from the comparator output controls a voltage on thecontrol terminal.
 5. The termination control circuit of claim 4, furthercomprising a second counter coupled between the comparator and thecontrol terminal.
 6. The termination control circuit of claim 5, whereinthe second counter increments or decrements in response to the signalfrom the comparator output.
 7. The termination control circuit of claim6, wherein the second counter stores a count that determines a currentthrough the adjustable resistor.
 8. The termination control circuit ofclaim 6 instantiated on an integrated circuit device with a driver todrive a signal from the integrated circuit device via a signal pad,wherein the second counter stores a count that establishes a drivecurrent through the driver.
 9. A termination control circuit comprising:a first node to receive a first reference voltage; a second node toreceive a second reference voltage; an adjustable resistor exhibiting anadjustable impedance responsive to an impedance-control signal; acomparator having a first input coupled to the first node to receive thefirst reference voltage, a second input coupled to the second node viathe adjustable resistor, and a comparator output; a transistor disposedin a path between the second input and a supply terminal, the supplyterminal to receive a third reference voltage; and a counter operativelycoupled between the comparator output and the adjustable resistor, thecounter to update the impedance-control signal responsive to a signalfrom the comparator output, wherein updating the impedance-controlsignal adjusts the impedance between the second node and the secondinput of the comparator; and termination elements coupled to the counterto receive the impedance-control signal; wherein the termination controlcircuit is instantiated on an integrated circuit device with a receivercircuit including a differential amplifier to receive a pair ofcomplementary signals via respective first and second signal pads, andwherein each of the signal pads is coupled to a DC voltage via arespective one of the termination elements.
 10. An integrated circuitdevice comprising: a receiver circuit including a differential amplifierto receive a pair of complementary input signals via respective firstand second nodes; a first termination element, exhibiting a firstimpedance, coupled to the first node and including a first impedancecontrol input; a second termination element, exhibiting a secondimpedance, coupled to the second node and including a second impedancecontrol input; and a control circuit having: an adjustable referenceelement to develop a reference impedance; and a feedback path to apply acalibration signal to the adjustable reference element to set thereference impedance; wherein the feedback path is coupled to the firstand second impedance control inputs to set the first and secondimpedances.
 11. The integrated circuit of claim 10, wherein the controlcircuit includes a comparator having a first input coupled to a firstnode to receive a reference voltage, a second input coupled to a secondnode via the adjustable reference element, and a comparator outputcoupled to an input to the feedback path.
 12. The integrated circuit ofclaim 11, further comprising a transistor coupled between the secondinput and a supply terminal.
 13. The integrated circuit of claim 12,wherein the transistor selectively disables current flow through thereference element.
 14. The integrated circuit of claim 12, wherein thetransistor includes a control terminal operatively coupled to thecomparator output.
 15. The integrated circuit of claim 14, wherein thecontrol terminal is operatively coupled to the comparator output via asignal path comprising a latch.
 16. The integrated circuit of claim 11,wherein the control circuit includes a counter to store the calibrationsignal.
 17. The integrated circuit of claim 16, further comprising alatch between the comparator output and the counter, the latch tocapture an output from the comparator.
 18. The integrated circuit ofclaim 17, wherein the counter updates the calibration signal in responseto the captured output from the comparator.
 19. The integrated circuitof claim 10, further comprising a first circuit pad coupled to the firstnode and a second circuit pad coupled to the second node, and a thirdcircuit pad coupled to the adjustable reference element.
 20. Theintegrated circuit of claim 10, further comprising a signal drivercoupled to the control circuit to receive the calibration signal, thesignal driver to provide a drive current proportional to the calibrationsignal.
 21. The integrated circuit of claim 20, wherein the controlcircuit further comprises a counter coupled to the feedback path tostore a count that determines the drive current.